weird c function syntax

I know my C up to this point. I was looking at the source files of PHP I downloaded, and I saw this strange syntax:

PHPAPI int php_printf(const char *format, ...)
{
// code...
}

What does the

PHPAPI

do before the return type

int

? I've tried searching all over and I can't understand what this means. Is it a second return type? It can't be because the function does return an int. Maybe it extends to some other struct declared in a header file?